feeding related: Nipple stings, causes?

I'm not sure if this is it, but if a baby doesn't latch well it can make your nipple sore. As babies get older, they'll sometimes switch up their nursing style and then this happens. Be sure that your baby is latching far back and getting a real mouthful. If s/he doesn't latch well the first time, break the suction and try it again until s/he gets it right. It can help to gently pinch the side of your areola and push the nipple far back into the baby's mouth to encourage a good latch.